1. Field of the Invention
The present invention pertains to the observation, by laser and incoherent or non coherent detection of widespread fluid masses and to their characterization through the compilation of the parameter measurements taken in several areas of these masses. It particularly pertains to the measurement of atmospheric activities (such as wind speeds) on Earth and other planets.
2. Description of the Prior Art
Traditionally, a "lidar" or Light Detection and Ranging, which equivalent, as one skilled in the art knows, to radar in the field of optic wave lengths (generally, wavelengths of between 0.1 and 15.mu. or more), is used for the measurement of atmospheric activities. A lidar embodies a laser transmitter and a receiver including one or several telescopes equipped with a detection assembly to analyze the signal sent by a target illuminated by the laser beam; this target may be reflective, in the case of a telemetric or altimetric lidar, or diffuse in the case of atmospheric lidars.
The measurement of the atmospheric activity is taken in practice by coherent detection, using a mobile target indicator (also referred to as "DOPPLER lidar" in some cases), which means that, by comparing the frequencies of a local oscillator and the radiation received, one determines the amplitude of the component, based on the transmission reception direction, of the local wind speed.
In fact, several cases of coherent detection may be distinguished, depending on whether the shot transmission laser (or part thereof) is used as a local oscillator (coherent homodyne detection), or if a second laser is used as a local oscillator (heterodyne reception).
Such DOPPLER lidars are, among others, used for the "LAWS" project of the National Aeronautics and Space Administration (NASA), the "ALADIN" project of the European Space Agency (ESA), or "BEST" of the Centre National des Etudes Spatiales (CNES), all of which pertain to wind speed measurement by satellite coherent detection.
A lidar placed on board of a mobile craft in altitude (such as an airplane or, better yet, a satellite) allows for the exploration of the atmosphere over a wide area (which may encompass all of the Earth in the case of a polar orbiting satellite): this area is mainly covered by the motion of the mobile craft possibly combined with the scanning capabilities of the lidar. The two types of scanning presently used are: the cross-scanning, (perpendicular to the speed vector of the mobile) and the cone scanning around the nadir axis (axis connecting the mobile with the center of the Earth). In the case of the DOPPLER-based lidars designed to measure the wind speeds, the cone type scanning helps define two wind speed components (and, therefore, determine the orientation and amplitude of the wind speed), each area being scanned at a few seconds interval along two distinct sight axes. The cross scanning technique does not define two such components. It is generally admitted that the vertical component of the wind speed is insignificant, this explains why only two measurements, taken along two distinct sight axes, are required to reconstruct the wind vector.
This is shown for example on FIG. 1 which schematically illustrates the aforementioned LAWS project concept, (the ALADIN concept being still under development).
This figure represents a space platform 1, orbiting in this instance at an altitude of between 400 and 800 km, and shown at its t.sub.o time position. This platform includes, mounted on its lower face, a telescope 2 with a sight axis V--V and equipped with a telescope bracket 3 connected with the space platform by means of a rotary motor schematically represented at 4 and whose Z--Z axis is parallel to the nadir. The V--V sight axis on the telescope is tilted at an angle of .theta., the order of 45.degree., with reference to the Z--Z motor axis, thereby defining a Z--Z axis cone with a half angle 0 relative to the platform, while its print on the ground and within the ground strip being observed is a spiraling curve whose course is indicated under reference C.
If P is the ground point intercepted at time t.sub.o by the sight axis V--V, it is possible to measure, at the precise moment, the component of the wind speed above P along the V--V axis: this particular component is referenced v.sub.o. The point P, or any point in the vicinity thereof, is again intercepted at a later time, at time t.sub.1, upon which the sight axis V--V has become V'--V' due to the combined platform motion, V speed, and cone-scanning action of the telescope: at this time t.sub.1, the component of the wind speed, referenced V.sub.1, along the V'--V' axis has an orientation sufficiently different from V.sub.o to allow, in combination, for the determination of the horizontal wind speed (it being assumed in practice that, during the time interval separating the measurements V.sub.1 and V.sub.o, the wind speed does not vary by much). It should be noted that, for clarity, the curve C is not represented at the same scale as that of the platform, the width L of the explored strip representing tens or hundreds of kilometers, whereas the coils of this curve are, when the scanning period is short enough (as few as tens of seconds in the case of a polar orbit), much closer to one another and intertwined with one another, which allows for a full coverage of the strip observed.
It should be noted at this point that the laser transmission is not continuous, as it consists of a series of brief shots, typically lasting between 10.sup.-12 and 10.sup.-5 secs. each, usually sent at a low 10.sup.-15 frequency, typically at about several microsecond intervals (knowing that safety standards applying to the energy of each shot must be observed in order to avoid the hazardous glare in the eyes of an individual standing on the ground and who might be hit by such a shot).
FIG. 2 schematically illustrates the "BEST" project concept. The platform, referenced 1', is equipped in this case with four fixed telescopes 2a, 2b, 2c and 2d symmetrically arranged with reference to the plane of the orbit (sharing the same plane with V and Z--Z) and whose sight axes V.sub.a --V.sub.a, V.sub.b --V.sub.b, V.sub.c --V.sub.c and V.sub.d --V.sub.d, respectively, are tilted at the same angle (45.degree.) with reference to the vertical axis Z--Z, while their tilt angle with reference to one another is the same (90.degree.). With this concept, only two lines 1.sub.1 and 1.sub.2, parallel with the orbit, are explored.
In the various cases where the platform is a satellite, the speed relative to the Earth is significant, (about 7 km/s in the case of a sun synchronous polar orbit) and, when combined with a cone scanning action, adds an additional and significant DOPPLER delay between the transmission and the reception; this delay may vary, based on the angle between the platform speed and the instant orientation of the sight axis, as illustrated on FIG. 1 (its value being maximum when the sight axis is included in the same plane containing speed vector V and axis Z--Z, and nil when the sight axis is in a plane perpendicular with the speed). The maximum value may vary with the scanning angle since it results from (2V/.lambda.).multidot.sin 0 (where V is the satellite speed, .lambda. the transmission wavelength, 0 the half-angle of the scanning cone), which produces a frequency shift of .+-.1to 10 GHz for a wavelength comprised between 1 and 10.mu., the most usual range for lidars, and a 45.degree. scanning angle.
The weak return signal received explains, in several cases, why a coherent detection method is used as opposed to a direct detection method: its complexity notwithstanding, it is the only technique which offers performances meeting the scientific requirements in the case of wind speed measurements taken from space (with a virtual accuracy of approximately one meter per second).
The DOPPLER frequency shift, however, presents a disadvantage in the case of coherent detection: the output signal of the detector, wherein the electric field produced by the reflected wave and the one produced by the local oscillator, used as a reference, are superposed, is proportional to [k(v.sub.s -v.sub.o1)], where v.sub.s and v.sub.o1 respectively represent the reflected signal frequency and the local oscillator frequency. It has been stated that this signal may vary from .+-.1 to 10 GHz during scanning at a 45.degree. angle; however, in order to attain accuracy on a frequency variating over such a large range, good dynamics are required and which are not at all compatible with the dynamics offered by the present detectors (or those presently under development). This frequency shift must be compensated for prior to the reception, either on the transmitting laser or on the local oscillator, which seems a priori easier since the requirements applying to the oscillator, frequency stability excepted, are generally less demanding than those imposed on the transmitting laser. However, this compensation makes the reception channel (or the transmission channel, if the laser and the local oscillator share a common oscillator) all the more complex. A 1 m/s accuracy translates into a frequency accuracy on the order of 1 to 10 MHz for a wavelength comprised between 1 and 10.mu. and a scanning angle of 45.degree.: the compensation error should be therefore less than 1/1000 of the compensation itself.
In the case of the direct detection (without comparison to a local reference), a problem is likewise encountered due to the fact that the reception filters must be sensitive enough (in the aforementioned case and where wind speeds reach about 100 m/s, the sensitivity range is less than a few tens of MHz) on a variation range of 1 to 10 GHz due to the DOPPLER of the platform: the incompatibility of these requirements requires the tuning (which may vary depending on the scanning angle) of one or all receiving filters.
No simple solution has, therefore, been found to date to meet these accuracy objectives.
It should be noted that if the satellite borne wind speed measuring lidar projects call for cone scanning and coherent detection (such as "LAWS" by NASA and "ALADIN" by ESA), using a single telescope, the "BEST" project developed by CNES calls for fixed telescopes, which eliminates the variable tuning problem but, as seen above, there is an adverse effect on the coverage of the explored areas (without the scanning it is only possible to cover two lines in parallel with the satellite course) and the total mass and volume of the instrument.